There is a known transmission system in which a plurality of transmission devices is coupled to a control device. In the transmission system, each of the plurality of transmission devices transmits and receives data to and from an external network and also transmits and receives the data to and from another transmission device. When transmitting and receiving the data to and from the other transmission device, each of the plurality of transmission devices performs clock synchronization with the other transmission device. Therefore, generally in the transmission system, a Synchronous (SYNC) board that supplies a common clock to the plurality of transmission devices is allocated.
FIG. 8 is a diagram illustrating an example of the transmission system in which the SYNC board is allocated. In the conventional transmission system illustrated in FIG. 8, three transmission devices are coupled to the control device through a Back Wiring Board (BWB), and the SYNC board is coupled to the three transmission devices. The control device supplies the three transmission devices with various clocks for control and controls in such a way that the three transmission devices perform various processing such as alarm collecting or switching. Each of the transmission devices performs various processing according to an instruction from the control device. The SYNC board supplies the three transmission devices with the common clock. As a result, each of the three transmission devices supplied with the common clock by the SYNC board may accurately transmit and receive the data to and from the other transmission device by using the common clock. As a related art, for example, Japanese National Publication of International Patent Publication No. 2008-507050 is disclosed.